Mineral lubricating oils and other functional fluids have a tendency to become thin and lose lubricating properties at elevated temperatures, and thicken at low temperatures, thus causing undesirable friction and drag on moving parts. The art has found it generally necessary to incorporate additives to improve the viscosity-temperature coefficient of lubricating oils. A useful class of such additives to improve the lubricating compositions with respect to viscosity index, while maintaining thickening properties and shear stability, are the ethylene/.alpha.-olefin polymers. Since representative patents disclosing such applications include the following U.S. Pat. Nos.: 3,389,087; 3,507,636; 3,522,180; 3,551,336; 3,563,964; 3,598,738. These copolymer materials, as well as polymers containing additional olefin and diolefin comonomers, have found extensive use in these applications. In the processing of these ethylene/.alpha.-olefin polymers in lubricating oils and the like, the polymers dissolved in the oil are often exposed to excessive temperatures for long periods of time, often in the presence of oxygen or air and agitation, which may result in a loss of desirable physical properties of the polymers for use as viscosity additives for lubricating oils, or in subsequent uses. Stabilized compositions having improved resistance to the effect of heat and air or oxygen are desired.